(i) Field of the Invention
This invention relates to aqueous suspensions of antimicrobial agents and antimicrobial hair treatment compositions in which fine particulate polyvalent metal salts of 2-mercaptopyridine-N-oxide (which may be hereinafter referred to simply as Mept compounds) having a specific size distribution are stably dispersed.
(ii) Description of the Prior Art
Polyvalent metal salts of 2-mercaptopyridine-N-oxide are known to be effective as antimicrobials and are widely utilized not only as ordinary antimicrobials, but also as anti-dandruff agents and are incorporated into shampoo compositions, hair rinse compositions and the like. The polyvalent metals of the Mept compounds are magnesium, barium, strontium, zinc, cadmium, tin, zirconium and the like. Of these, zinc salts are widely used.
However, these Mept compounds are sparingly soluble in water, for instance, the zinc salt (hereinafter referred to simply as Zpt compounds) has a solubility in water of 15 ppm at 25.degree. C. Accordingly, when they are incorporated into shampoo or hair rinse compositions, they must be utilized in a form of dispersions.
However, because of the considerable difference between the specific gravities of Zpt compounds (specific gravity =1.8) and a medium liquid for dispersion, the Zpt compounds are apt to settle and separate as time passes, and thus it was difficult to obtain a stable dispersion system containing Zpt compounds.
In order to prevent such settlement or separation, the following methods are known:
(i) Make the static viscosity of the dispersion medium high.
(ii) Make the particle size of Mept compounds so small that Brownian movemett will be dominant in the system.
By either method, settlement will be made difficult to take place.
Examples of method (i) are a method of adding viscosity increasing polymers such as cross-linked polyacrylates (Japanese Patent Publication No. 49-49117) and a method of adding acrylic acid/acrylate copolymers (Japanese Patent Publication No. 54-16951). However, these methods have the drawback that limitation is placed on the type of surface active agent usable to stably disperse Mept compounds.
As for method (ii), since Mept compounds having very small particle size are difficult to be produced, compositions containing fine particulate Mept compounds stably dispersed therein have not been practically obtained.
Under such conditions, the present inventors formerly made a study and succeeded in manufacturing Mept compounds having very small particle size (hereinafter referred to as fine particulate Mept compounds) compared with conventional Mept compounds incorporated into shampoo or hair rinse compositions (Japanese Patent Application Nos. 58-122845, 58-122846 and 59-82690).
The present inventors made further study in order to obtain stable antimicrobial suspensions making use of thus obtained fine particulate Mept compounds, and found that fine particulate Mept compounds are very sensitive to conditions of liquid media for dispersion and are apt to coagulate, leading to difficulty in keeping the original size distribution. More specifically, the following phenomena were noted.
(1) Fine particulate Mept compounds in a suspension coagulate when electrolytes such as salts are included in the suspension, or when the suspension is got frozen or heated.
(2) Fine particulate Mept compounds are apt to coagulate when they are incorporated into hair treatment compositions such as shampooes o hair rinses in which surfactants are contained, because the surfactant per se is an electrolyte.
In order to prevent the coagulation of fine particulate Mept compounds, surface modification or colloidal protection have been proposed.
Examples of adding a water-soluble polymer to a shampoo composition which incorporates Mept compounds are disclosed in Japanese Patent Publication Nos. 47-20635 and 50-22044, in which cationic polymers are used as a water-soluble polymer in order to enhance the adsorption of Mept compounds to the hair and head skin. However, the cationic polymers function as a coagulating agent for fine particulate Mept compounds having a specific size distribution, and produce considerable coagulation.
The method of forming colloid layer of cellulose-type water-soluble polymers for protection also has a drawback in that the cellulose polymers coagulate when they form a colloidal adsorption layer on a particle to be protected. For instance, additinn of an aqueous solution of hydroxyalkylcellulose or the like, which is referred to in Japanese Patent Application Laid-Open Nos. 53-14710, 53-14711, 53-97010 and 57-176906, results in losing the original size distribution, leading to losing the stable dispersability.
From this reason, conventional art was directed to make use of cellulose-type water-soluble polymers as a thickner to enhance the viscosity of shampoo base thereby allowing the incorporated fine particulate Mept compounds to settle more slowly, or further incorporating swelling clay or pearling agent in order to give structual viscosity to the system of the shampoo composition, thereby preventing settlement of Mept compounds. Such methods cannot avoid limitations to surfactants to be incorporated into a shampoo base or viscosity of the shampoo base.
Preparation of hair rinse compositions or hair conditioning compositions is also accompanied by the limitation to viscosity of the composition in the case where fine particulate Mept compounds are incorpoarted into the compositions. Namely, the bases are limited to a highly viscous ones such as of emulsion system or gel.